Some achievements have been made in the researches on the treatment of flow-back fluid of fracturing fluid produced in the shale gas recovery process in China and foreign countries. Specifically, the majority of inorganic wastes in the flow-back fluid, such as rock cuttings, proppants, and inorganic salts, etc., can be effectively removed through a series of physical settlement and chemical flocculation processes, etc. However, polymers that are hard to degrade in the flow-back fluid, such as high molecular polymers, e.g., guar gum, etc., have to be degraded one by one at the cost of heavy energy consumption or consumption of various kinds of chemical agents, involving drawbacks including high treatment cost, high equipment investment, long treatment cycle, and possible secondary pollution, etc.
Photocatalysis techniques are new techniques developed in the late stage of the twentieth century for treating degradation-resistant organic pollutants, and are characterized in that highly active hydroxyl radicals (.OH) are produced under the action of a semiconductor catalyst. The hydroxyl radicals (.OH) can almost non-selectively oxidize and degrade organic pollutants that are hard to degrade in waste water into non-toxic or low-toxicity small molecular substances, or even directly mineralize them into carbon dioxide, water and other small molecular carboxylic acids, to attain the purpose of innocent treatment. These techniques have advantages including non-selectivity, strong oxidization ability, high reaction velocity, high treatment efficiency, and free of secondary pollution, etc. Owing to those advantages, photocatalysis techniques exhibit a great application space in the treatment of shale gas flow-back fluid. In the field of photocatalysis techniques, the development of photocatalyst is one of the core techniques. Bismuth oxyhalide has become a new favorite in the photocatalysis field in recent years.
As a new narrow bandgap semiconductor, bismuth oxyhalide has become a focal point in the photocatalysis field in recent years. Owing to the characteristics such as nontoxicity, cheapness, strong oxidation-reduction capability, stable chemical properties, and photocorrosion resistance, etc., bismuth oxyhalide has been widely applied in water pollutant degradation and anti-bacteria aspects, etc. However, the high electron-hole recombination efficiency of bismuth oxyhalide monomer has adverse effects to the photocatalytic performance of bismuth oxyhalide, and severely limits the actual application of bismuth oxyhalide in photocatalysis, etc. Therefore, it is one of important subjects in the photocatalysis research field to improve the photocatalytic performance of bismuth oxyhalide-based photocatalysts through modification. Up to now, the reported methods include metal ion doping, non-metallic ion doping, semiconducting solid solution and compounding, etc.
In 2016, Zhang et al. found that in-situ synthesized BiOIxBr1-x solid solution photocatalyst had stronger photocatalytic activity than the monomer when the photocatalyst was used to catalyze Rhodamine B under irradiation of visible light (Xing, Z., Wang, C. Y., Wang, L. W., Huang, G. X., Wang, W. K., & Yu, H. Q. (2016). Fabrication of BiOBrxI1-x photocatalysts with tunable visible light catalytic activity by modulating band structures: Scientific Reports 6, 22800.).
Though the BiOIxBr1-x solid solution photocatalyst reported in the above-mentioned document was prepared with the simplest precipitation method, the performance of the photocatalyst itself was still low, and the utilization of light energy was still not ideal. Further modification is required to improve the inherent photocatalytic performance of the photocatalyst.
Therefore, seeking for an economic, effective, and environment friendly polymer degradation technique or process for flow-back fluid of fracturing fluid is especially important for alleviating the environmental problems in the development blocks and of far reaching importance for ensuring normal production and sustainable development of shale gas.